This invention relates to an electronic watthour meter including a conversion element, such as a magneto-electric conversion element, for conversion into electric energy by the use of a physical effect.
A conventional electronic watthour meter is disclosed, for example, in Japanese Patent Publications Nos. 58508/1984 and 54704/1985. The conventional electronic watthour meter comprises an auxiliary voltage transformer for producing a voltage output signal proportional to a load voltage of a distribution line, an auxiliary current transformer for producing a current output signal proportional to a consumed current, a power-voltage conversion circuit for multiplying the voltage output signal and the current output signal from the auxiliary voltage transformer and the auxiliary current transformer to produce an output voltage proportional to a load power, a voltage-frequency conversion circuit supplied from the power-voltage conversion circuit with the output voltage proportional to the load power for integrating the output voltage to produce a pulse signal, a frequency dividing circuit for weighting the pulse signal from the voltage-frequency conversion circuit to produce a divided pulse signal proportional to electric power, and a display circuit responsive to the divided pulse signal from the frequency dividing circuit for displaying the amount of electric power consumption represented by the divided pulse signal.
FIG. 1 shows a structure of the power-voltage conversion circuit including a Hall element (Hall-effect element) as a magneto-electric conversion element. Referring to FIG. 1, an auxiliary voltage transformer 1 produces a voltage output signal proportional to a load voltage. The voltage output signal from the auxiliary voltage transformer 1 is converted through a resistor 2 into a current i which is supplied to a Hall element 3. A consumed current supplied to an auxiliary current transformer flows through a primary conductor 5 wound around a core 4 to produce a magnetic field proportional to the consumed current. The magnetic field is applied to the Hall element 3.
Supplied with the current i and the magnetic field having a field intensity B, the Hall element 3 produces, by a Hall effect, a voltage Vop proportional to electric power corresponding to a product of the current i and the field intensity B, as represented by the following formula: EQU Vop.varies.kiB,
where k represents a proportional constant.
The voltage Vop produced by the Hall element 3 has a low level. As shown in FIG. 1, the voltage Vop is amplified by an amplifier circuit 6 and integrated by an integrator circuit 7 of the voltage-frequency conversion circuit to be produced as a voltage signal having an appropriate level proportional to the electric power.
Generally, the Hall element 3 produces an offset voltage even when no input is supplied, namely, when no magnetic field is applied. This results in deterioration of a linear characteristic of the Hall element 3. In particular, considerable deterioration is observed in case of a small load.